Fluidized bed processes are known to provide superior heat and mass transfer characteristics as contrasteel with fixed bed processes. They permit substantially isothermal reactor conditions in conducting both exothermic and endothermic reactions.
For example in the production of synthesis gas (hydrogen and carbon monoxide), low molecular weight hydrocarbons, e.g., natural gas (primarily CH.sub.4), are fed into the bottom of a reactor containing a mixture of catalyst, e.g. a nickel-on-alumina catalyst, and a solids diluent, e.g., alumina, to form a fluidized bed of the catalyst and the solids diluent. Steam is introduced into the reactor. Oxygen is fed into the fluidized bed through nozzles separate from those through which the natural gas is fed. The oxygen reacts with a portion of the natural gas in a zone near the oxygen inlet according to the following partial oxidation reaction: EQU (1) CH.sub.4 +O.sub.2 .dbd.CO+H.sub.2 +H.sub.2 O (Partial Oxidation)
This is a strongly exothermic reaction and produces localized hot spots and burning near the O.sub.2 nozzle, or nozzles, the high temperature area around the O.sub.2 nozzle constituting a burning zone. The natural gas that does not react directly with the O.sub.2 ascends through the reactor where it undergoes a steam reforming reaction to produce hydrogen and carbon monoxide according to the following equation: EQU (2) CH.sub.4 +H.sub.2 O.dbd.CO+3H.sub.2 (Steam Reforming)
The steam reforming is highly endothermic, but by having good solids circulation in the fluidized bed, the overall bed temperature becomes quite uniform. The net, or overall reaction (the sum of reactions (1) and (2), supra), described as follows, is slightly exothermic. EQU (3) 2CH.sub.4 +O.sub.2 .dbd.2CO+4H.sub.2 (Overall)
The overall reactions occur in a net reducing atmosphere.
The water gas shift reaction also occurs in the bed, a very rapid reaction which produces only minor heat effects. EQU (4) CO+H.sub.2 O.dbd.CO.sub.2 +H.sub.2 (Water Gas Shift)
The exothermic heat of reaction produced by the oxygen causes burning and severe localized heat near the oxygen inlet zone; and despite the good heat transfer in the fluid bed, the high temperature produces net agglomeration of the catalyst, or catalyst and other solids. The high localized flame temperature produced by the oxygen in the burning zone of the bed can exceed the melting point of the alumina, or at least produce temperatures which cause the surface of the alumina particles to melt, stick and fuse together as the particles repetitively collide or recycle through the burning zone of the bed. The amount of agglomeration increases with time which adversely affects the fluidization characteristics of the bed, and the activity of the catalyst generally declines. The active catalytic sites can become inaccessible to the reactants due to the agglomeration. In addition, the overall increase in the average particle size of the fluidized bed produces larger bubbles in the bed; a phenomenon which causes further drop in the CH.sub.4 conversion due to increased mass transfer resistance. As a result thereof the quality of fluidization becomes increasingly poorer and fluid bed temperatures become increasingly non uniform; qualities which decrease the amount of CH.sub.4 conversion. In addition, reactor vibration can increase due to the poor fluidization characteristics; a phenomenon which can lead to a loss in the mechanical integrity of the equipment.